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Bleaching May Have Killed Half the Coral on the Northern Great Barrier Reef, Scientists Say.

SYDNEY, Australia — Mass bleaching on the Great Barrier Reef in the past three months has killed as much as half of the coral in the north but left large parts of the southern reaches with only minor damage, scientists in Australia said on Sunday.

The current bleaching is the third to strike the roughly 1,400-mile-long reef in 18 years and the most extreme scientists have recorded.

“In the north, the mortality rates are off the scale,” said Prof. Terry Hughes, the director of the ARC Center of Excellence for Coral Reef Studies, based at James Cook University in Townsville, Queensland. “There, the coral mortality rates are approaching 50 percent, and the impact of the bleaching is still unfolding.”

But from Cairns, in tropical north Queensland, southward down the east coast of the state, about 95 percent of the coral has survived, Professor Hughes said. Mildly bleached coral should regain its color over the next few months, although the stress from bleaching is likely to slow the area’s reproduction and growth, he said.

Bleaching occurs when water temperatures rise as little as 1 degree Celsius, or 1.8 degrees Fahrenheit. The coral then expels tiny, colorful algae, causing it to turn white. The coral can recover if the water temperature drops and the algae, known as zooxanthellae, recolonize it. Otherwise, it may die.

Scientists diving on the reef in March and April recorded an average death rate of 35 percent for bleached coral in the north and central parts of the reef. Mass bleaching reduces the disparity between corals that might survive and those that are more vulnerable, resulting in higher death rates across the reef, Professor Hughes said.

Another scientist, Verena Schoepf from the University of Western Australia, said portions of the reef off the Kimberley coast had suffered severe but patchy bleaching and death rates of about 15 percent so far in the current bleaching event. The Kimberley region is across the far north of the state of Western Australia.

“We are seeing these events occur so close together, due to global warming, that coral does not have time to recover,” Professor Hughes said. “Some of the large, 50- to 100-year-old corals we saw on the very northern parts of the reef are now dead. We won’t see them there again.”